In a case of a storage apparatus using such a storage media as a magnetic disk, a magneto-optical disk and an optical disk, the storage media is accessed based on a request by a data processing apparatus. In order to read/write the storage medium in a desired unit, it is necessary to format on the storage media.
This formatting can be physical formatting, such as CKD formatting, and a logical formatting which separates the data area in the processing units of the host OS (e.g. in block units) and provides identification. Because of this formatting of a storage media, the storage media can be accessed (e.g. read/write) using a host command of the host OS.
The physical format processing means the initialization processing for a media in track units, and the logical formatting means the format processing for the data area of physically formatted tracks in block units. For the logical format processing, it takes time to format (initialize) all the blocks for the media. Hence in order to actually use an unformatted media, the host must wait for the completion of format processing for a long time (e.g. over 30 minutes).
To solve this problem, a method for executing format processing of a storage media, not as an initialization processing, but in the background of processing a host I/O that is accepted, has been proposed (e.g. Patent Documents 1, 2 and 3).
According to Patent Document 1, for example, two types of methods are executed for this format processing (called “quick format processing”). The first format processing (called “one-point format processing”) executes RAID formatting by host I/O processing when a host I/O, that includes an area where RAID formatting has not been completed, is received, and the second format processing (called “sequential format processing”) sequentially executes RAID formatting from the beginning to the end of a RAID group.
FIG. 15 is a diagram depicting conventional one-point format processing. A RAID (Redundancy Array Independent Disk) device 200 connected to a host 100 has a channel adapter (CA) 202, a control module 204 and a plurality of physical disk drives 210, 212 and 214.
Here a configuration of RAID 5 (2+1), which is comprised of two data disk drives and one parity disk drive, is shown. The control module 204 receives an I/O request from the host 100 via a channel adapter 202. The control module 204 judges whether the range of this I/O request has been RAID-formatted or not. If the requested range has been RAID-formatted, the control module 204 continuously processes the I/O request (read or write) from the host.
If RAID-formatting has not be executed, on the other hand, the control module 204 executes RAID formatting with a stripe width (size) of each disk drive, then executes the I/O request. FIG. 15 shows a case of the physical disk drives 210 to 214, where formatted areas AF and unformatted areas UF coexist.
In FIG. 15, if the I/O request is a read request for data #5 of the physical disk 212, the storage stripe (area) of data #5 is an unformatted area UF, therefore format data Parity #2, Data #5 and Data #4, having the stripe widths of each disk drive 210, 212 and 214, are created respectively on the data buffer 208. And each of these data is written in areas having a same stripe number in each disk drive 210, 212 and 214. Thereby RAID formatting completes.
Then data #5 in the physical disk 212 is read and staged in the cache memory 206. The data #5 in the cache memory 206 is then transferred to the host via the channel adapter 202. In this way, the I/O request from the host is executed.
In the case of the sequential format method, on the other hand, RAID formatting is executed sequentially on the disk drives 210, 212 and 214, from the beginning to the end, according to the internal schedule, when an I/O request has not arrived for a predetermined time. If an I/O access is arrived in an area which has not yet been formatted by the sequential format processing, this area is formatted first for the I/O request. This method is called the “one-point format”.
The time required for the RAID formatting is directly added to the I/O response time, so it is not a good idea to increase the size (area) of the RAID formatting. Hence conventionally the RAID format size of the one-point format and that of the sequential format are the same. And this size is set to a size with which the delay of the host I/O response does not increase to the extreme.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-29933    Patent Document 2: Japanese Patent Application Laid-Open No. 2003-241904    Patent Document 3: Japanese Patent Application Laid-Open No. 2005-11317
Recently the capacity of a disk drive is increasing because of the release of large capacity disk drives (e.g. 500 GByte or 750 GByte disk drives). If the above mentioned RAID format size is used for a RAID group constituted by such large capacity disk drives, a number of times of format writing processing per one disk drive increases. And this tends to increase the RAID formatting time.
If the RAID format size is increased to prevent this problem, delay of the host I/O response increases, which drops I/O processing performance.